Suggestions on how to test a BufferedReader and FileReader that takes in strings and puts them into an ArrayList

Question

I have a class which has a method that reads a text file line by line and then puts each line into an ArrayList of Strings. Here is my code:

public class ReadFile {

    public List<String> showListOfCourses() throws IOException {
        String filename = "countriesInEurope.txt";
        FileReader fr = new FileReader(filename);
        BufferedReader br = new BufferedReader(fr);
        List<String> courseList = new ArrayList<>();

        while (true) {
            String line = br.readLine();
            if (line == null) {
                break;
            }

            courseList.add(line);
        }
        br.close();

        return courseList;
    }

}

I was hoping for a few suggestions on how to test this method through Mockito involving Arrange/Act/Assert. I have heard readers which involve text files can be tricky to test and that creating a temporary file for is not best practice as it uses up memory? Any suggestions will be really appreciated.

Answer 1

As the filename countriesInEurope.txt is hardcoded in your implementation, this is not testable. A good way to make this testable would be to refactor the method to take a Reader as parameter:

public List<String> showListOfCourses(Reader reader) throws IOException {
    BufferedReader br = new BufferedReader(reader);
    List<String> courseList = new ArrayList<>();

    // ...

    return courseList;
}

Your main implementation could pass a FileReader to this. On the other hand when testing, your test method could pass a StringReader instance, which is easy to create with a sample content as a simple string, no temporary files needed, for example:

@Test
public void showListOfCourses_should_read_apple_orange_banana() {
    Reader reader = new StringReader("apple\norange\nbanana");
    assertEquals(Arrays.asList("apple", "orange", "banana"), showListOfCourses(reader));
}

Btw the name of the method is not good, as it doesn't "show" anything. readListOfCourses would make more sense.